I assume by "stand out", what you are really after is a way to quickly distinguish top from bottom. The best way to do this is to use different contrast and totally opposite large geometric shapes. For example, a burst on top color, black and white invasion stripes on the bottom. So totally different you can’t confuse the two, and very quick to detect, i.e. really don’t have to even think about it. So for you yellow wing bottom, how about just a large dark stripe, front to back, out about 2/3 of the span and about ¼ span wide. One left, one right. I did this on all of my pattern ships and it works well. As for color, just pick something dark in contrast, any color will work, red, blue, black, whatever, as long as it is dark in contrast, it will stand out on the yellow.